This invention relates to a friction controller for reducing the noise in a fluid flow system in which the flow moves in one direction.
Valve noise is becoming more of a concern because of the increased interest in environmental safety. Numerous valves have been developed over the last 30 years in a attempt to either suppress or control noise produced by different fluids in transit. The term fluid, as herein used, refers to either a liquid or a gas. In the earlier devices, valves and the surrounding flow channels were simply wrapped with acoustical materials for absorbing sound. Later designs involved attenuators which were placed directly in the flow stream to either divide the flow into smaller sub-streams or shape the flow into a desired geometry. Although these devices provided limited noise suppression under controlled conditions, they failed to address all the factors that trigger noise producing vibrations in a flow system. These factors can be either fluid related or mechanical in nature. The fluid related factors include such things as cavitation, flow turbulence and fluid separation in and about flow boundary regions. Mechanical factors, which are typically caused by obstructions in the flow path include such things as shock related stresses and transient vibrations.
Two approaches have been proposed for combatting these factors. The first is directed towards eliminating or avoiding noise producing conditions. The second involves attenuating the noise once it has been produced. Most noise suppression devices follow the latter approach and include such things as static mixes, tube bundles, filters, flow separators, dampers, and diffusers. All attenuators have some type of structure that disrupts the normal flow of the fluid thereby producing their own design problems. Generally, fixed attenuators can handle, only a limited range of frequencies and flow noise can be produced outside this frequency range.
Variable or tunable attenuators have been devised which can be adjusted to accommodate changes in frequency produced at different flow rates. The tunable attenuator is typically made a part of a flow control valve stroke mechanism and is automatically adjusted in response to changes in the valve stroke. Examples of such variable tunable attenuators includes cascade chambers, stacked discs, nested cylinders, parallelly aligned resistive elements and tortuous flow paths.
Although the tunable attenuator will reduce noise under varying flow conditions, the amount of noise suppression that is attainable is still rather limited. These devices oftentimes produce changes in the velocity profile of the flow as the stroke is being adjusted which, in turn, produces pressure disturbances and discontinuous flow patterns. Typically, the variable attenuator will include several orifices or chambers that are partially opened or closed to selectively distribute the flow as it moves through the valve stroke. High flow velocities can thus be generated under certain operating conditions. This, in turn, will cause cavitation and separation along the flow boundaries, producing unwanted noise.
Other problems associated with tunable attenuators are design related. The attenuator represents an extra part in the flow system and generally requires a significant amount of additional mounting space. The attenuation system is not only bulky, but is also expensive to manufacture and difficult to assemble and maintain. Attenuator flow passages tend to be restricted and therefore are easily clogged with contaminants unless high filtration requirements are met.
In U.S. Pat. No. 4,150,696 to Meier et al., three separate approaches for achieving improved noise suppression are proposed. The first approach involves placing some type of obstruction within the flow stream to divide the main flow into smaller laminar subflows. Wire grids and the like are used for this purpose. The second approach relates to changing the geometry of the main flow stream. In this embodiment, a deformable elongated screen is placed downstream from a thin edge orifice. Air is either blown through the screen or suction applied thereto to alter the flow pattern as the fluid passes through the screen region. Lastly, Meier et al. suggests shifting the frequency of the flow noise into a range that is outside the audible range. It should be noted that all of the Meier approaches require the use of flow interrupting devices that are inserted directly into the flow path. Accordingly, these devices disturb the flow and create unwanted flow related problems.
Hayes et al., in U.S. Pat. No. 4,003,405, discloses a device for maintaining laminar flow conditions within a flow system. The device consists of a circular casing having a continuous inlet that encircles the outer periphery of the casing. The entering fluid is directed inwardly towards the center of the casing where it is abruptly turned 90.degree. before being discharged through a relatively small diameter discharge pipe. The circular flow passage within the casing converges rapidly as it approaches the central discharge opening. The passage functions as a restriction to flow which reduces the flow to a point where it remains laminar within the casing. Although the flow through the Hayes et al. casing may be laminar, noise can still be generated, particularly where the flow turns into the restricted discharge region due to cavitation and/or flow separation at the system boundaries.